Aluminum hydroxide (alum) is the most common coagulant used in water and wastewater treatment. The main purpose of alum in these applications is to improve the settling of suspended solids and color removal. Alum is also used to remove phosphate from wastewater treatment effluent. In all these applications, aluminum enriched sludge is generated. Alum in the sludge generated during these processes causes difficulties with sludge dewatering and disposal because alum traps water. Therefore, aluminum recovery from water and wastewater treatment sludge has several significant advantages: (1) improving sludge dewaterability, (2) reducing sludge mass or weight by dewatering the sludge, and (3) cutting down the cost of alum by reusing the recovered alum.
There are limited options for recovery of alum from sludge containing high concentration of CaCO3, CaSO4, and Mg(OH)2. The most widely accepted method for aluminum recovery is acidulation of the sludge. Exemplary examples of aluminum recovery under acidic circumstance include CN105948442, TW201226577, KR20040088093, U.S. Pat. Nos. 5,304,309, 6,495,047, US2013319941, JP2002113472, RU2133225, U.S. Pat. Nos. 4,130,627, 3,959,133, etc.
Attempts have been made to recover aluminum hydroxide under alkaline conditions. For example, EP2017225 to Olsson et al. discloses a process for production of an aluminum hydroxide containing filter cake from aluminum containing waste waters, in which acidic and alkaline waste waters containing aluminum are mixed in a ratio resulting in a pH of 9-11 to obtain mixed waste waters. The mixed waste waters are allowed to react with each other during a retention time of at least 5 hours resulting in an aluminum hydroxide containing slurry, which is dewatered to obtain an aluminum hydroxide containing filter cake.
EP2452924 to Rossi et al. discloses a method for the treatment of waste liquids originating from plants for the treatment of aluminum that includes mixing a first alkaline eluate (B) that contains sodium hydroxide (NaOH) and sodium aluminate (NaAlO2), and a second acid eluate (A) which contains sulfuric acid (H2SO4) and aluminum sulfate (Al2(SO4)3) to achieve the forming of an aluminum hydroxide precipitate (Al(OH)3) on the bottom of a reactor upon reaching a pH value of the content of the reactor between 7.5 and 9.0.
Masschelein et al. (W. J. Masschelein, R. Devleminck and J. Genot, “The Feasibility of Coagulant Recycling by Alkaline Reaction of Aluminum Hydroxide Sludges”, Water Research, vol. 19, No. 11, 1985, p 1363-1368) disclose recovery of aluminum from a waterworks sludge by alkalization with NaOH or Ca(OH)2 under laboratory conditions. Even though it was possible to recover 80% of the aluminum the authors recommended recycling less than 50% of the total dosing rate because of impurity problems. Lime was more effective than sodium hydroxide for removing heavy metals from the recovered coagulant.
In addition, cost is also an issue to be considered for aluminum hydroxide coagulant recovery industry.
Thus, a need remains for effective recovery of aluminum hydroxide from wastewater treatment sludge.